This invention relates to fiber filled composites, and more particularly to thermoplastic, organic fiber-filled thermoplastic composites that have a high resistance to mold.
There is a very high demand for wood products. Although wood is a renewable resource, it takes many years for trees to mature. Consequently, the supply of wood suitable for use in construction is decreasing and there is a need to develop alternatives.
Organic fillers including lignocelluosic or cellulosic fiber materials such as wood, sawdust, rice hulls, and the like have long been added to thermoplastic resins such as polyethylene, polypropylene and polyvinyl chlorine (PVC) to achieve a wood-like composite providing reinforcement, reduced coefficient of expansion, and cost reduction. Process methods have been developed to enable blends containing materials having low bulk density (ie. powders) and poor flow characteristics to be fed at commercially acceptable rates. Blends of this type can be extruded through dies of appropriate configuration to produce building product type shapes previously made from wood. When these thermoplastic composites were first introduced, the prevailing theory was that the plastic protected the fibers from fungal attack. However research by Verhey, Laks, and Richer, described in “Laboratory Decay Resistance of Woodfiber Thermoplastic Composites”, Forest Products Journal, September 2001 revealed that fiber-filled thermoplastics are susceptible to damage from fungal decay. Degradation due to the fungal attack is a problem that threatens the material's structural integrity. In contrast, surface discoloration and spotting has been reported shortly after the introduction of thermoplastic composites. This visual degradation, caused by mold, is a significant problem since major commercial uses of organic fiber-filled thermoplastic composites, including decking and fencing, rely on their aesthetic appeal to compete in the marketplace. The problem is intensified since these products are directly exposed to the elements for years.
Traditionally, solid wood products are dipped or pressure treated with solutions of fungicides to provide resistance to fungus and mold damage. While this type of treatment is not practicable for a thermoplastic product, it is possible to incorporate a fungicide into the product during its manufacture. This approach provides a constant loading of fungicide throughout the material's thickness, increasing the resistance to leaching of the fungicide from the composite. However it diminishes surface concentration of the fungicide, reducing its effectiveness against surface mold attack.
Anhydrous borax, zinc borate and calcium borate have been identified as resisting fungal decay at relatively low levels, typically less than 1.5 percent, in both lignocellulosic compounds formed from small fractions of wood bonded with an adhesive binder of phenol-formaldehyde resin and thermoplastic lignocellulosic composites. Patent application Ser. No. 10/681,497 describes their use to resist surface impairment caused by mold a levels starting at 2 percent by weight. However there is a need to control the surface impairment in fiber-filled thermoplastic composites caused by mold more effectively at lower borate loading levels.